1. Field of the Invention
The present invention relates to a process for modifying synthetic silica powder so that, when the modified powder is fused and vitrified at high temperatures to form a quartz glass product, the quartz glass product will have reduced amounts of carbon and hydroxyl groups and fewer bubbles. The present invention also relates to quartz glass products made using the modified synthetic silica powder.
2. Discussion of the Background
Synthetic silica powder can be produced by a sol-gel method by drying and baking a gel produced by hydrolyzing a metal alkoxide. The resulting synthetic silica powder is amorphous and of high purity with few metal impurities than natural quartz powder. However, this synthetic silica powder is known to contain residual carbon and hydroxyl groups from the alkoxide. When a quartz glass crucible is produced from synthetic silica powder made by the sol-gel method, the residual carbon and hydroxyl groups can cause unwanted bubbles to form in the quartz glass.
Many attempts have been made to reduce the residual carbon in the synthetic silica powder produced by the sol-gel method. For example, Japanese Laid Open Patent Nos. H09-86916 and No. H10-287416 disclose a process in which the dried gel powder is heated to remove residual carbon by burning at a temperature of less than 600° C., where the pores in the powder are not closed, and then, after the decarbonization, baking and vitrifying the powder at a temperature of from 1000° C. to 1300° C., where the pores in the powder are closed, to produce amorphous synthetic silica powder. A process is known in which silica powder produced by the sol-gel method is heated at about 500° C. in air to burn organic components, and then is vitrified by heating to a higher temperature to remove the OH groups. Japanese Patent Registration No. 2530225 discloses a process in which the dried gel powder is baked in two steps in a dry atmosphere or a vacuum at the time of making the synthetic silica powder to produce synthetic silica powder having a predetermined high density. However, it is extremely difficult using these conventional processes to reduce to a large extent the residual carbon in the synthetic silica powder. As a result, it is very difficult to produce the quartz glass products having few inner bubbles.
On the other hand, Japanese Patent Laid Open No. 2001-261858 discloses a quartz crucible produced by a process in which quartz powder produced by the sol-gel method is heated in a hydrogen atmosphere at a temperature of from 800° C. to 1300° C. until the quartz powder no longer exhibits a spectral peak indicating O═C═O bonding. However, the hydrogen reacts with hydroxyl groups in the quartz powder to produce adsorbed water that forms inner bubbles when the powder is vitrified. Moreover, the process is dangerous because the hydrogen gas is introduced at the high temperatures. Furthermore, Japanese Patent Laid Open No. 2001-89168 discloses heating fumed-silica produced by the sol-gel method, and then baking the fumed-silica at high temperatures in a vacuum, a hydrogen atmosphere or a helium atmosphere to make quartz glass powder.
Although all of these conventional processes are aimed at modifying quartz powder by removing residual carbon, the cause of the bubbles created when the quartz powder is vitrified is not limited to the residual carbon. The air components adsorbed on the quartz powder after the heat treatments also become an important cause of the inner bubbles.